The present invention relates generally to voice and data communications over digital subscriber lines and, more particularly, to a method and apparatus for improving the performance of a splitterless Asymmetric Digital Subscriber Line (ADSL).
While telephone lines are useful for data communications using modems, the data rates that can be achieved over ordinary telephone lines are rather limited. Attempts have been made to re-engineer existing telephone lines for digital data communications. These include Integrated Services Digital Network (ISDN) and, more recently, Digital Subscriber Lines (DSLs). Since most computer users tend to receive more data than they send, Asymmetric Digital Subscriber Lines (ADSLs) having different bandwidths for reception and transmission have been favored over regular DSLs.
In order to maximize the utility of the existing copper cable plant, ADSL has been implemented to allow an ordinary POTS (xe2x80x9cplain old telephone servicexe2x80x9d) communication channel to coexist with the ADSL data communication capacity. However, originally, ADSL required a POTS splitter to be installed at the point where the ADSL enters a business or residence. The POTS splitter separated the POTS communication channel from the ADSL data communication channel allowing ordinary telephone instruments to receive/transmit over the POTS communication channel without the need for any sort of adapters.
Unfortunately, a certain amount of specialized knowledge is required in order to properly install the POTS splitter. Thus, a version of ADSL known as splitterless ADSL or G.Lite was developed. Splitterless ADSL avoids the need for a POTS splitter at the customer premises. However, since there is no splitter to separate the ADSL data from the POTS voice signals, the ADSL signal is present at telephone jacks throughout the subscriber""s home or business. This ADSL signal may become corrupted if incompatible telephone instruments are plugged into the telephone jacks.
The G.Lite variant of ADSL has also been referred to as universal ADSL, by the Universal ADSL Work Group (UAWG). One of the goals of UAWG is to develop an ADSL technology that allows simple modem installation by the end user. This refers to G.Lite""s promise to allow telephone companies to provision ADSL and allow customers to set up G.Lite DSL connections on their ownxe2x80x94without the delay and expense of a telephone company service call.
The standard for G.Lite arises from a series of technical requirements which were approved at the October, 1998, meeting of the ITU (International Telecommunications Unionxe2x80x94a group headquartered in Geneva, Switzerland). The ITU""s Study Group 15 Q4 called the technology G.Lite. The ANSI (American National Standards Institute) T1/E1.4 committee and the ADSL Forum (an industry group) call it splitterless ADSL or Lite ADSL.
The original installation concept in ADSL required a centralized POTS splitter to be installed on the telephone line usually at the NID (network interface device). The purpose of this POTS splitter is to split the frequency range into two isolated bandsxe2x80x94a telephone (POTS) band and an ADSL band.
The implications of the above installation concept are that installation of the POTS splitter is non-trivial and that there is a need to install new wiring from the NID to the ADSL modem. As a result, installation of ADSL will not be readily installed by end users. It will require, in most cases, the assistance of the telephone company.
The G.Lite modem uses the existing home wiring (used today for POTS), so the installation becomes simple. G.Lite and POTS operate together on the same internal home wiring system, allowing the customer to plug both a telephone and a modem into a standard wall telephone outlet.
When a phone goes off hook, noise is generated from the phone in the same frequency range as the ADSL signal (because of non-linear effects). Furthermore, the impedance of an off hook phone may be so low that it essentially shunts the strength of the ADSL signal. The increase of noise and decrease of signal will usually cause errors in the ADSL signal, which will require a re-start process in order to re-sync. The opposite can happen as well. The ADSL signal can sometimes be heard as an annoying sound in the telephone.
A small lowpass filter (inline filter) may be installed in series with the telephone apparatus to filter out the interference between the data signals and the voice signals. The inline filter behaves as a distributed POTS splitter, i.e., instead of installing a splitter at the NID, as with splitter-based ADSL modems, the customer may instead install an inline filter on the most problematic phones, or even on all phones.
When the inline filter (low-pass filter) is installed between the wall jack and the telephone, it isolates the phone from the modem. The phone will not influence the modem and the ADSL transmit signals will be filtered out before entering the phone.
In order to cope with the disruptive interference of phones on the ADSL signal and the annoying interference of ADSL signals into a telephone when an inline filter is not used, the modem must retrain when one of the phones goes off hook. This fast retrain takes about one or two seconds and allows the modem to adapt itself to the new condition.
During the fast retrain, the service stops and is reestablished afterwards. This interruption is undesirable and is considered to be one of the main disadvantages of G.Lite. Putting the phone back on-hook requires another fast retrain routine since the off hook settings of the modem are not optimal for the on-hook case.
G.Lite modem installation involves the use of at least one inline filter in the majority of homes. Inline filtering may be required in order to make G.Lite work at fast bit rates or in some cases to allow it to work at all.
A significant percentage of corded phones require an inline filter in order to meet minimum standards for audible interference in the off hook condition. However, some cordless phones do not require an inline filter and behave as if an inline filter is already installed inside. In both cases (phones with inline filters and some cordless phones), fast retrain for the modem is not always a necessity when phones go off hook or return to on-hook.
However, the presence of certain telephones requires a modem to retrain when the telephones transition between on-hook and off hook states. Thus, a technique is needed to distinguish efficiently between situations where retraining is needed and where it is not.
The present invention provides a method and apparatus for improving the performance of splitterless ADSL systems. While ideally users would install lowpass filters on all telephones when using splitterless ADSL, it can be expected that users will neglect to do so in some cases. Some telephones, when switching between on-hook and off hook states, will cause interference with ADSL data, while others will not. The present invention provides a uniform technique for adapting an ADSL system to any of these situations. The invention provides a graduated approach to handling anomalies that may be caused by incompatible telephone equipment.
In accordance with one embodiment of the present invention, the hook state information and in line filter indication is communicated from the ADSL Transceiver Unit-Remote side (ATUR) to the ADSL Transceiver Unit-Central Office side (ATUC). This indication informs the ATUC whether a phone, not having an ILF, has been taken off hook, hereinafter referred to as an xe2x80x9cOffHook-NILFxe2x80x9d indication. The dynamic properties (i.e. behavior over time) of the measured SNR in splitterless ADSL systems varies significantly between on hook and off hook states. In particular, when a phone not having an ILF has been taken off hook, the SNR varies significantly, e.g. approximately 35 db to approximately 40 db, over time as function of the voice signal introduced into the mouthpiece. The OffHook-NILF indication is therefore used to determine robust bits and gain allocation during startup and also to correct the profile selection during fast retrain. In an alternative embodiment, the xe2x80x9cOffHook-NILFxe2x80x9d indication may be detected by the ATUC side and communicated to the ATUR.
In an alternative embodiment, instead of determining whether the telephone that has been taken off hook has an associated in line filter, it may be assumed that the telephone does not have an associated in line filter, whereby the OffHook-NILF indication is used to account for expected noise fluctuation.